K3 Li3 Gd7 (BO3 )9 : A New Gadolinium-Rich Orthoborate for Cryogenic Magnetic Cooling

Reversible single-crystal-to-single-crystal transition in Gd(III) metal-organic frameworks induced by heat and solvents with a significant magnetocaloric effect

The design and synthesis of a Gd(III) metal-organic framework with the formula [Gd4(BTDI)3(DMF)4]n (JXUST-40, H4BTDI = 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)diisophthalic acid) are reported hererin. Interestingly, a reversible single-crystal-to-single-crystal transition between JXUST-40 and {[Gd4(BTDI)3(H2O)4]·6H2O}n (JXUST-40a) was achieved under the stimulation of heat and solvents. Both JXUST-40 and JXUST-40a exhibited good stability when soaked in common solvents and aqueous solutions with pH values of 1-12. Magnetic studies showed that JXUST-40a has a larger magnetocaloric effect with -ΔSmaxm = 26.65 J kg-1 K-1 at 2 K and 7 T than JXUST-40 due to its larger magnetic density. Structural analyses indicated that the coordinated solvent molecules play a crucial role in the coordination environment around the Gd(III) ions and the change in the framework, ultimately leading to the changes in the pore size and magnetism between JXUST-40 and JXUST-40a. In addition, both isomorphic [Dy4(BTDI)3(DMF)4]n (JXUST-41) and {[Dy4(BTDI)3(H2O)4]·6H2O}n (JXUST-41a) displayed slow magnetic relaxation behaviour.

Gd3 TeBO9 : A Rare-Earth Borate with Significant Magnetocaloric Effect

Magnetic refrigeration technology based on Gd-based paramagnets is expected to be applied to refrigeration in extremely low temperatures, thereby reducing the consumption of liquid helium. Here, we obtained a compound, Gd3 TeBO9 with high Gd3+ concentration through element substitution. The Gd3+ concentration in this compound is as high as 2.4×1024  ions/kg, which is 33 % higher than the commercial Gd3 Ga5 O12 (GGG), and further magnetic tests show that Gd3 TeBO9 has a large magnetic entropy change (57.44 J/(kg K) and 408 mJ/(cm3  K) at 2.6 K and 7 T), which is 1.5 times that of GGG, implying the possibility of its further development as an potential magnetocaloric material.

Sr14.06Gd14.63(BO3)24: A Gadolinium-Rich Borate with Magnetic Refrigeration Performance

A single crystal of Sr_14.06Gd_14.63(BO₃)₂₄ has been successfully grown through a high-temperature solution technique with K₂O-KF-B₂O₃ as the flux. It crystallizes in the Pnma space group with parameters a = 22.3153(5) Å, b = 15.9087(4) Å, c = 8.7507(2) Å, and Z = 2. Sr_14.06Gd_14.63(BO₃)₂₄ has a three-dimensional (3D) framework built from [GdO] chains, in which the isolate [BO₃]^3- groups and Sr²⁺ ions fill in the space of the 3D framework. The magnetic measurements revealed that the title compound exhibits a large magnetocaloric effect with the magnetic entropy change of -ΔS_m = 42.2 J kg^-1 K^-1 at 2 K for 7 T, which is higher than that of the commercial material, Gd₃Ga₅O₁₂ (GGG), with -ΔS_m of 38.4 J kg^-1 K^-1 under the same conditions. Moreover, the infrared spectrum (IR), UV-vis-NIR diffuse reflectance spectrum, and thermal stability were investigated.

Gd(OH)F2: A Promising Cryogenic Magnetic Refrigerant

Magnetic refrigerants with a large magnetocaloric effect (MCE) in a wide temperature range and low magnetic ordering temperature (T_o) in the sub-kelvin temperature region are not only crucial for adiabatic demagnetization refrigeration but also open up a broader parameter space for the optimal design of adiabatic demagnetization refrigerators. However, such magnetic refrigerants are extremely rare because they require magnetic materials to simultaneously satisfy three conditions: low T_o, weak magnetic interaction, and high magnetic density. Here, we report the syntheses, heat capacities, and magnetic properties of Gd(OH)_3-xF_x (1: x = 1, 2: x ≈ 1.5, and 3: x = 2), demonstrating for the first time that the introduction of fluoride anions into antiferromagnetic Gd(OH)₃ can effectively regulate its T_o. Significantly, 3 not only has a T_o of 0.5 K but also exhibits a large MCE in the temperature range from 0.5 to 4 K, representing the best magnetic refrigerant reported to date in the temperature range of 0.5-4 K from the viewpoint of the MCE.

Gd(III)-based metal-organic frameworks and coordination polymers for magnetic refrigeration

As the alternatives of expensive and increasingly shortage 3He for ultralow-temperaturerefrigeration, molecule-based magnetorefrigerant materials have attracted much attention in the past decades. Among them, Gd(III)-based metal-organic frameworks and coordination polymers...

Giant and reversible low field magnetocaloric effect in LiHoF4 compound

Cryogenic refrigeration technology is gradually penetrating into increasing aspects of human life and industrial production. Magnetic refrigeration shows excellent application potential due to its high efficiency, good stability and environmental friendliness. It is important for a magnetic refrigeration system to secure a high-performance magnetocaloric material under a low applied magnetic field, which can greatly simplify the design and reduce the expense. In this study, LiHoF₄, a polycrystalline compound prepared by an improved solid-state reaction method undergoes a second-order phase transition below 2 K. Meanwhile, an unexpected giant low-field magnetocaloric effect has been observed. The maximum magnetic entropy changes are 11.0 J kg^-1 K^-1, 19.0 J kg^-1 K^-1, and 25.9 J kg^-1 K^-1 in field changes of 0.6 T, 1.0 T, and 2.0 T, respectively. The giant and reversible low field magnetocaloric effect proves it to be one of the most practical candidates among the cryogenic magnetic refrigerants.

Large Magnetocaloric Effect in Li3K9Gd3(BO3)7 Crystal Featuring Sandwich-Like Three-Dimensional Framework

A new Gd-based borate crystal, Li₃K₉Gd₃(BO₃)₇, has been successfully obtained via the high-temperature solution method using Li₂O-K₂O-B₂O₃ self-flux. It crystallizes in monoclinic space group P2/n (no. 10) with lattice parameters a = 11.3454(6) Å, b = 9.9881(4) Å, c = 11.4467(7) Å, α = γ = 90 ^o, β = 114.782(7) ^o, and Z = 2. Li₃K₉Gd₃(BO₃)₇ exhibits an intriguing sandwich-like three-dimensional (3D) framework constructed from [Gd-B-O]_∞ layers, KO_n (n = 6 and 8) polyhedra, and LiO₄ tetrahedra, in which [Gd-B-O]_∞ layers are built from two types of GdO₈ polyhedra and triangular BO₃ units. Magnetic measurements showed that Li₃K₉Gd₃(BO₃)₇ exhibits a large magnetocaloric effect with -ΔS_m = 39.3 J kg^-1 K^-1 at 2.0 K for ΔH = 7 T, which is slightly higher than that of the commercial gadolinium gallium garnet under the same conditions. The powder X-ray diffraction, infrared spectrum, and UV-vis-NIR diffuse reflectance spectrum were also performed to characterize Li₃K₉Gd₃(BO₃)₇. The electronic band structures, partial density of states, and refractive indices of Li₃K₉Gd₃(BO₃)₇ were investigated via the first-principle calculations.

KLi2RE(BO3)2 (RE = Dy, Ho, Er, Tm, Yb, and Y): Structural, Spectroscopic, And Thermogravimetric Studies on a Series of Mixed-Alkali Rare-Earth Orthoborates

We report a detailed structural, spectroscopic, and thermogravimetric investigation of a new series of mixed-alkali rare-earth orthoborates KLi₂RE(BO₃)₂ (RE = Dy, Ho, Er, Tm, Yb, and Y). Single crystals were directly prepared by a flux method as well as mechanically separated from the polycrystalline powder obtained from the conventional solid-state reactions. All KLi₂RE(BO₃)₂ members are isotypic and crystallize in the space group P2₁/n. The novel structure type is comprised of [RE₂(BO₃)₄O₄]^14- anionic clusters where the edge-sharing REO₇ pentagonal bipyramids are connected by BO₃ groups and both K⁺ and Li⁺ cations are located at the interstitial voids of the 3D network. The metric parameters and crystal structural features obtained from the single-crystal data are in excellent agreement with those refined from the powder data. The change of the lattice parameters and unit cell volumes can be explained in terms of the lanthanide contraction effect. A comparison between KLi₂RE(BO₃)₂ and other rare-earth borates with similar chemical compositions indicates that the sum of the ionic radii of the alkali-metal cations governs the symmetry of the crystals. Diffuse reflectance UV-vis spectra display the characteristic absorption behaviors of the RE³⁺ cations and the fundamental absorption edge. Both the Tauc's and derivation of absorption spectrum fitting (DASF) methods were used to identify the magnitude and type of bandgap, respectively, which are compared with those obtained from density functional theory (DFT) calculations. The calculated phonon density of states and the vibrational frequency at the gamma point help explain the Fourier transform infrared and Raman spectra of KLi₂RE(BO₃)₂. The incongruent melting behavior and the thermal stability of each member of the KLi₂RE(BO₃)₂ series were also studied by thermogravimetric analyses.

Gadolinium-Rich Borate Gd17.33(BO3)4(B2O5)2O16 Exhibiting a Magnetocaloric Effect

A gadolinium-rich borate Gd_17.33(BO₃)₄(B₂O₅)₂O₁₆ was successfully grown by the high-temperature solution method using the Rb₂O-B₂O₃ flux. The crystal crystallizes in centrosymmetric space group C2/m with lattice constants a = 18.4300(2) Å, b = 3.7400(4) Å, c = 14.2047(16) Å, and β = 119.8550(12)°. Two different honeycomb-like [GdO] layers alternately arrange in the order ABAB forming the three-dimensional framework, in which the isolated [B₂O₅] and [BO₃] units fill in channels of the 12-membered and 10-membered [GdO] polyhedral rings, respectively. The UV cutoff edge of Gd_17.33(BO₃)₄(B₂O₅)₂O₁₆ is less than 240 nm. The maximum -ΔS_m,max is 26.53 J kg^-1 K^-1 or 174.70 mJ cm^-3 K^-1 (T = 4.4 K and ΔH = 7 T) as investigated by the isothermal magnetization method.

A new non-centrosymmetric Gd-based borate crystal Rb7SrGd2(B5O10)3: growth, structure, and nonlinear optical and magnetic properties

A new non-centrosymmetric borate crystal, Rb7SrGd2(B5O10)3, was successfully grown via the spontaneous nucleation technique from the Rb2O-B2O3-SrO self-flux system. It crystallizes in the trigonal system space group R32 with lattice parameters a = b = 13.4975(5) Å, c = 15.3223(8) Å, α = β = 90°, γ = 120°, and Z = 3. Its three-dimensional framework is composed of isolated GdO6 and SrO6 octahedra, RbOn (n = 6 and 8) polyhedra, and [B5O10]5- clusters. Rb7SrGd2(B5O10)3 exhibits multifunctional properties, and has both moderate second harmonic generation (SHG) responses (0.5 × KDP) and paramagnetic characteristics with μeff = 8.18μB. Thermal stability, Fourier transform infrared spectroscopy, and UV-Vis-NIR diffuse reflectance spectroscopy were performed to characterize the title compound. Its electronic band structures and density of states were also investigated via first-principles calculations.

Li3Ba4Sc3(BO3)4(B2O5)2: featuring the coexistence of isolated BO3 and B2O5 units

A new mixed alkali/alkaline earth scandium borate Li₃Ba₄Sc₃(BO₃)₄(B₂O₅)₂ with isolated and almost co-planar BO₃ and B₂O₅ groups is reported.

High-Performance Magnetic Refrigerant Featuring One-Dimensional Gd-O Chains and O-Gd3 Triangles

Magnetic cooling at low temperature has attracted intensive interest in cryogenics research, which may become important as cooling medium for long-wave photon detectors to support space exploration. Here, we report a Gd-based quaternary magnetic refrigerant material, Gd₅ BSi₂ O₁₃ , containing chains of face-shared GdO₉ polyhedra and geometrically frustrated OGd₃ triangles. Magnetic measurements indicate that Gd₅ BSi₂ O₁₃ exhibits a large magnetocaloric effect (MCE) about 1.74 times that of the practical magnetic refrigerant GGG (-ΔS_m =67.0 J kg^-1  K^-1 ). We analyzed the origin of the large MCE by comparing Gd^III -containing compounds with different structures and concentrations of Gd^III .